The present invention generally relates to an image transmitting system, and more specifically to an image transmitting system which is capable of transmitting dynamic image data for use in TV conference, video telephone, and so forth.
Generally, a typical image transmitting system comprises an information source converting section for converting input image data into a coefficient suitable for coding, a quantizing section for quantizing the coefficient, a variable-length coding section for generating a binary code word including xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d by variable-length-coding the quantized coefficient, a dequantizing section used for an information source conversion which uses a correlation between frames, and a buffer for smoothing the generated amount of code for transmission to a line with a particular fixed speed.
FIG. 1 is a diagram showing a conventional image transmitting system, which consists of an image data conversion block 1, a quantization block 2, a variable-length coding block 3, a dequantization block 10, a smoothing buffer 16 and a quantization controlling section 7.
The image data conversion block 1 converts input image data into a coefficient suitable for coding. The quantization block 2 quantizes the coefficient having been converted at the image data conversion block 1. The variable-length coding block 3 variable-length-codes the quantized coefficient in order to generate a binary code word including xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d.
The dequantization block 10 is used for an information source conversion which uses a correlation between frames. The smoothing buffer 16 smoothes the generated amount of code for transmission to a line with a particular fixed speed. The quantization controlling section 7 operates quantization parameters in order to control the generated amount of code.
In the above-described conventional image transmitting system, changing a line speed is accomplished by changing a coding speed. Therefore, when a low speed line is selected, the generated amount of code per unit period has to be kept small, which consequently causes coarse coding of the quantized coefficient, resulting in reproducing a possible blurred image.
Accordingly, when one attempts to send an image of a document at a TV conference, etc., while selecting a low-speed line, for instance, it may be a problem that the transmitted image is blurred and the contents of the document become unrecognizable or difficult for reading.
The present invention has been achieved in order to solve such problem in the prior art, and it is an object of the present invention to provide an image transmitting system which is capable of having unchanged image resolution even when the line speed is set to a low-speed.
In accordance with the present invention, an image transmitting system comprises a plurality of line interfaces connected to lines with different line speeds, and image-codes data for transmission under the same resolution without having to depend on the selected line speed.
In other words, the image transmitting system of the present invention has a scalability structure on the time base. In the present invention, instead of having an image resolution (space resolution, fineness) changed at the time of selecting a line with a lower speed, a resolution (smoothness) on the time base is changed.
In accordance with the present invention, in the image transmitting system, an input image is regarded as blocks, and a single block contains a plurality of frames arranged on the time base. In the image transmitting system, data conversion on a first frame is executed internally closed within a block; and data conversion on a second frame and frames beyond that is executed using a correlation between a subject frame and a frame before that. In this way, the image transmitting system classifies the input data into different kinds of image data in terms of frames, and executes quantization and variable-length coding on each kind of data. Then, the classified image data are inputted to different smoothing buffers, respectively.
In accordance with the present invention, on the basis of the selected line speed, the image transmitting system decides the absolutely necessary amount of generated code. Then, it is determined at which rate the generated amount of code should be transmitted to each smoothing buffer. At this time, since the coefficient generated after the closed conversion within the frame with respect to the first frame is more important than the second frame and the frames beyond that, a bit allocation is set larger. In other words, the average amount of generated code to be inputted to the smoothing buffer is set larger.
In this case, as to the optimum method of allocating the amount of generated code, there is an allocation method in which quantization distortion is set to a minimum, on the basis of a statistical distribution of coefficients obtained after the information source conversion. Among possible combinations of such allocation, the present invention connects with the smoothing buffer associated with the first frame when selecting the slowest line, with the smoothing buffers associated with the first and the second frames when selecting the line of a second lowest speed, and with the smoothing buffers associated with the 1st, 2nd, . . . Nth frames when selecting the line of Nth lowest speed.
In connecting to a plurality of smoothing buffers, switching of output of the smoothing buffers is executed in response to a detection of a break (i.e. a picture header when applying MPEG (Moving Picture-Coding Experts Group)) in a single frame worth of code within a smoothing buffer.
Thus, in the present invention, the coding speed does not decrease even when the line speed is decreased. Consequently, even when the line speed is set low, it is possible to always have the same image resolution without having a space resolution of each frame decreased.